You can check here for your best options for a free antivirus: http://www.pcmag.com/article2/0,2817,2388652,00.asp (hope the link's allowed, if not just google PC magazine and then search for "free antivirus" on their page).
If you are using Windows, I've heard (but couldn't vouch) that Microsoft's free antivirus (you can download it from the Microsoft official website) is good enough for basic users.
We use Norton, so I don't have any personal experience with free antiviruses, but I do use the above mentioned magazine's recommendations for lots of other free software and never was disappointed.
Also, do invest in an external HD, and make a regular backup to it of all your important stuff (there is also a setting in Windows that will do it automatically for you if you program it to) - keep in mind that if you have lots of things to back up (like your photos) then this takes up a considerable amount of resources from the PC and also time, so you'd want to set it to do the backup for you when you can leave your PC at it and don't need to use it for much else.
AND: NEVER EVER OPEN LINKS FROM E-MAILS unless you are 200% sure of where they come from (so +1 on always checking the sender's actual e-mail address not just the name) and +1 on checking the link BEFORE CLICKING IT (just place your mouse over the link without clicking it and you'll see a small pop-up kind of window that shows you the entire address where the link will take you). Also, a TIP: if you do for whatever reason click on such a link, but start having immediately second thoughts, close your browser ASAP. It might help prevent the damage.
Also, before you get a chance of doing the damage control that others already suggested (checking your PC and installing some protection), I would NOT use it for sensitive stuff (like home banking, buying online, typing passwords, caming or whatever you wouldn't want to share with strangers). If you MUST use it, than at least use a safe browser (the privacy versions, all browsers have them, or even TOR) and the built-in screen keypad (instead of the keys on the laptop, even non-touch windows PCs have an onscreen keyboard you can just use to type with the help of the mouse) as these will make it difficult to the hacker to access your personal stuff!
Hope you get it all sorted out!